1. Field of the Invention
The present invention relates to a laser module which is constituted by one or more semiconductor laser elements emitting one or more laser beams, an optical fiber, and an optical injection system making the one or more laser beams converge at an end face of the optical fiber.
2. Description of the Related Art
Conventionally, the laser modules having the following construction are known as the pigtail-type laser modules in the field of optical communications. The pigtail-type laser modules are constituted by a semiconductor laser element being contained in a package and emitting a laser beam, an optical fiber being fixed to the package and having an end (light-entrance end) which appears inside the package, and an optical condensing system making the laser beam converge a light-entrance end face of the optical fiber.
In the case where the output power of a pigtail-type laser module is high, contaminants deposit on surfaces of optical components at which the optical power density is high, so that troubles such as lowering of the transmittance or breakage of the optical components can occur. In particular, low-molecular-weight siloxane being emitted from silicon-based material and suspending in air causes photochemical reaction with oxygen, so that SiO2 is produced and fixed to surfaces of glass components as if evaporation is performed on the surfaces, as indicated in Japanese Unexamined Patent Publication No. 11(1999)-054852 (hereinafter referred to as JPP 11(1999)-054852).
When the transmittance is lowered by the deposition of contaminants on an optical component, the output power of the laser module decreases. Therefore, the optical component in the laser module deteriorates faster than the semiconductor laser (which is the light source in the laser module), so that it is difficult to achieve long-term reliability.
Japanese Unexamined Patent Publication No. 2004-252425 (hereinafter referred to as JPP 2004-252425) and Japanese Unexamined Patent Publication No. 2004-253783 (hereinafter referred to as JPP 2004-253783) disclose techniques for preventing deposition of contaminants on an end face of an optical component. According to the technique disclosed in JPP 2004-252425, a light-entrance end face of an optical fiber is held in close contact with an outer surface of a glass window of a package. In addition, according to the technique disclosed in JPP 2004-253783, a light-entrance end face of an optical fiber is protected by hermetically sealing the light-entrance end face or holding a transparent member (such as a glass block) in close contact with the light-entrance end face.
However, the techniques disclosed in the above patent publications have the following problems.
JPP 2004-252425 discloses a structure wherein a receptacle capable of receiving a ferrule is fixed outside a hermetically sealed package containing semiconductor lasers and lenses, and an optical fiber the tip of which is inserted into the ferrule is connected to the receptacle by using a connector which can fit into the receptacle. However, since the semiconductor lasers and the lenses are contained in the hermetically sealed package, the size of the package increases. Generally, the hermetically sealed package is expensive, and the cost of the hermetically sealed package further increases when the hermetically sealed package is large. In addition, in the above structure, the ferrule is held in contact with the light-output window of the hermetically sealed package. Therefore, it is necessary that the glass window have a hermetically sealing function. Further, since the glass window is pressed when the ferrule is kept in contact with the glass window, it is necessary to increase the strength of the glass window and the sealing portion of the package in order to maintain the hermetically sealing function against the pressure. However, provision for increasing the strength of the glass window and the sealing portion further increases the total cost.
On the other hand, as mentioned before, JPP 2004-253783 discloses a structure for preventing deposition of contaminants on the light-entrance end face of the optical fiber by fixing a transparent member to the light-entrance end face or holding the transparent member in close contact with the light-entrance end face. Specifically, the transparent member is fixed to a chassis, a support, or the tip of the optical fiber. Therefore, it is difficult to replace the transparent member.